The general perception with
the public regarding the extinction of the dinosaurs is that they were wiped out
by a meteorite impact. This theory has inarguably become the most famous and highly publicized cause
for the disappearance of the species 65 million years ago. Its popularity has
caused it to be universally accepted as the only viable theory to explain the
disappearance of the dinosaurs.

In 1980, Luiz Alvarez, a Nobel prize
winning physicists, teamed up with his son Walter, a geologist, and presented the meteorite
impact theory. They had conducted research on clay layers found at the K/T boundary
in the Umbrian Apennines (Gubbio section) in Northern Italy, and then matched
their results to data from clay layers in New Zealand and Denmark. The Alvarez's
research studied abundances of rare earth elements (REE) in the clay layer.
They discovered anomalies for a few of the REE: although one stood out, a large positive anomaly in the
abundance of Iridium (Ir) in the clay.

Figure: Reproduction of the graph
presented by the Alvarez's in 1980.

Iridium is found in minute
quantities on the Earth's crust. Its average crustal abundance is ~0.3ppb (parts
per billion). In the upper mantle its further depleted; whereas its slightly
enriched in the lower mantle. However, its believed that the core is enriched in
Ir and other REE. In the clay layers in Italy, the Alvarez team found abundance
2 to 3 orders of magnitude higher than expected on the surface. They calculated
the abundance of Ir in the clay in Italy to be 9.0ppb or higher. This value did
not correspond at all with surface abundances of Ir. So the Alvarez's pinned the
cause on a meteorite impact. This was principally due to two reasons:

Ir is relatively abundant in meteorites.

The isotopic composition of Ir in the clay
represents that of a meteorite and not the crust.

They also found chromium isotopes which had
similar composition as carbonaceous chondrites.

The clay layer was dated to be 65Ma and it
corresponded to the extinction of animals which were observed in the
sedimentary. The Alvarez pinned the cause of the extinction on the meteorite
impact. This theory came to be known as the "Alvarez hypothesis".

Figure: (Left) Map showing the
location of the Chicxulub crater. (Right) A 3-D gravity model of the crater.
Since the crater is not a surficial feature, it was discovered using seismic
studies and geophysical methods (such as gravity and magnetic) which probed deep
into the subsurface.

There was one thing missing from the "Alvarez
hypothesis": an impact crater or site. The Alvarez's had shown that the
meteorite had to be 10km in diameter which would have created a ~150-200km wide
crater. The Alvarez had failed to find any craters that would have matched the
age or the size. Ten years later, Alan K. Hildebrand and Glen Penfield found it.
They discovered the Chicxulub crater site in the Yucatan Peninsula and dated it
to be around 65 Ma. The discovery of the impact site, along with debris in the
sedimentary record in North America, provided the proverbial "smoking gun" and
much needed credibility to the "Alvarez hypothesis."